For purposes of communication between telephone subscriber stations associated with separate and distinct telecommunication switching systems, "communication trunks" are utilized and defined as communication channels interconnecting the switching systems. A communication trunk comprises a transmission facility, and a controllable trunk interface circuit in each switching system which serves to establish communication and signaling paths between the transmission facility and the circuits of the switching system.
A modern electronic telephone switching system typically comprises a central processor which provides operational control of the communications trunk by transmitting control signals to the trunk interface circuit via signal distributing circuitry. Scanner output signals representative of information signals received from the other telecommunication system and of the operational state of the interface circuit are transferred to the central processor via scanner circuitry connected to the trunk interface circuit. Several problems exist in the prior art systems using conventional trunk interface circuits, in both electronic and electromechanical telephone systems. To accommodate communication with different types of systems, communication trunks of various classifications are required which has necessitated nonuniform trunk circuit architecture. This variance in trunk circuit design is alleviated somewhat in electronic telephone systems by the assignment of control of the trunk interface circuit to the central processor, which generates state signals which determine the operational states of the trunk interface circuit. However, such direct control of these operations requires extensive real-time processing, and is costly in both processing time and stored program complexity. Additionally, when an electronic telephone system is connected to a system having a unique method of signaling, additional central processing programs may be required and existing programs are likely to require extensive modification.
A further problem is particularly associated with "periodic pulse metering" (PPM) systems employed in telephone systems in several countries. "Periodic pulse metering" refers to a system which utilizes charging pulses transmitted to the telecommunication system associated with a calling subscriber station wherein the transmittal occurs during the duration of the call on a periodic basis. These pulses require periodic accumulation and other real-time intensive tasks to be performed by the telecommunication system associated with the calling subscriber station. Attempts have been made in the prior art to overcome this problem by means of complex logic circuitry residing in the trunk interface circuits. Such circuitry is costly and does not solve the problem of nonuniform trunk circuit architecture. Further, it requires nonuniform interface circuits between the central processor and the trunk interface circuits.